The invention generally relates to a turbine rotor blade. Preferably, it relates to one having an internally uncooled blade profile, which extends starting from the blade platform on which a blade root abuts. The blade root preferably engages in a turbine disk and has a radial cross section with a region of increasing width toward the blade platform.
In order to increase the efficiency and/or the effective cross section of gas turbines, the blade profiles of the turbine rotor blades are lengthened as much as possible in order, by this, to achieve a better utilization of the hot gases flowing past. This lengthening of the blade profile is, however, limited by a plurality of parameters.
Due to the lengthened blade profile and the increased mass moved because of it, a hub region of the turbine disk, for example, is severely loaded by the centrifugal force applied to it. An attempt is made to counter this by increasing the load-supporting area in the hub region by axially lengthening the disk. This lengthening possibility is, however, limited.
Due to the increased blade profile, it is not only the hub which is more severely loaded but also the region in which the turbine blades are inserted, via their roots, into retention recesses of the outer periphery of the turbine disk. A lengthening of the blade profile can also take place radially relative, to the disk hub. But, due to this, the retention recesses of the outer periphery would be brought closer together. Further, their distance apart would become smaller and, therefore, the disk region between them would be more severely loaded. This loading, however, can only be increased to a limited extent without risking damage to the turbine disk.
An object of an embodiment of the present invention is to create a turbine rotor blade which permits a lengthening of the rotor blade profile without an increase, or with only a slight increase, in local loadings on the turbine disk and/or rotor blade roots.
An object may be achieved by an embodiment of the present invention by a blade root having an open cavity, which faces away from the platform, having a blind ending at the platform end and having a cross section which widens in the region of increasing width of the blade root.
In order to secure strength, the blade root usually has a solid configuration and, in comparison to the other dimensions of the turbine blade, has a larger cross section. In consequence, its mass is high and represents a large proportion of the centrifugal force loading, which occurs during rotation of the turbine blades, on the turbine disk and on the retention fixtures for the blades. The cavity considerably decreases the mass of the root and therefore the centrifugal force load. The particular shape of the cavity, namely a widening of the cross section at the longitudinal walls in the region of increasing width of the blade root, ensures an optimum utilization of the shape of the blade root with respect to reducing the mass. Because the cavity has a blind ending at the platform end, the strength requirementsxe2x80x94which are very high particularly in the region between platform and blade profile due to numerous severe force and temperature effectsxe2x80x94are satisfied.
At the same time, therefore, an embodiment of the invention permits the mass of the blade to be kept small and its strength to be maintained or even improved. Due to the reduction in weight, the average stress level in the root region is lowered and stress peaks in the retention teeth of the root and the adjacent turbine disk are moderated, which leads to a lengthening of the life of the turbine blades and, in particular, to an improvement in the durability of the root. Without endangering the strength of the turbine blade and while retaining the shape of the root, it is therefore possible to lengthen the rotor blade profile outward and, by this means, to increase the efficiency of the turbine.
A satisfactory strength of the platform-end blade profile region is provided because the cavity ends in a transition region between the blade root and below the upper surface of the platform. The force effect on the blade is particularly high above the platform upper surface and the blade has a narrower configuration than it has in the platform region. If, however, the cavity ends below the upper surface of the platform, the force effect is, to a sufficient extent, accepted by the stable platform and the adjacent regions.
In order to avoid stress peaks and, therefore, locally excessively high loads, it is proposed that the cavity should be bounded by mainly rounded walls and should end in a vaulted shape below the platform upper surface.
A very large reduction in mass is provided by the fact that longitudinal walls of the cavity extend over practically the complete length of the blade root and transverse walls extend over almost the complete width of the blade root, the walls of the cavity ensuring sufficient strength when centrifugal force is applied.
The forces, the major part of which are applied centrally, are transmitted well in solid regions of the blade without loading the walls of the cavity too severely, if the cavity has its maximum height in the central region and falls away toward the transverse walls and longitudinal walls.
If regions of the longitudinal walls of the cavity widen continually on approaching the platform end while maintaining a minimum wall thickness at the transverse walls of the cavity, the weight reduction is improved and, at the same time, an abrupt change in the curvature, which leads to local stress maxima, is avoided on transition to a rounded end region.
The hot working gases particularly affect those edge regions of the blade which are the first to be directly subjected to the incident flow. Account is taken of the higher strength requirements of the hot gas incident flow end by the minimum wall thickness being greater in the vicinity of the hot gas incident flow end than it is at the hot gas outlet flow end.
An increase in the strength of the root, while economizing in material and mass, is provided by the blade root being reinforced by transverse struts configured between its longitudinal walls. The forces applied to a longitudinal wall of the cavity are transferred through the transverse struts to the other longitudinal wall of the cavity and through both walls to the turbine disk, without endangering the strength of the cavity. Due to the further reduction in the mass, furthermore, there is a further removal of load from the root because of the reduced centrifugal force load.
If the transverse struts are at a distance from the walls of the cavity at the platform end of the blade root and/or from the end facing away from the platform of the blade root, this provides an additional economy in weight while retaining the strength.
Optimum transmission of the forces takes place because positions and shapes of the transverse struts are matched to a force line path which occurs due to the application of centrifugal forces to the blade profile. By a matched number and shape of the transverse struts, it is therefore possible, on the one hand, greatly to reduce the mass of the blade root (because the walls of the cavity can have a thinner configuration due to the supporting effect of the transverse struts) and, at the same time, it is possible to maintain a homogeneous stress variation along the longitudinal sides of the cavity because of the support by the transverse struts.
The particularly high forces applied in the central region are accepted because the transverse struts of the cavity have their maximum height in the central region and decrease in height to match a fall-away in the shape of the cavity.